The present invention relates to an iron-based powder composition for powder metallurgy comprising an iron-based powder such as iron powders and alloy steel powders; an alloying powder such as graphite powder, and copper powder; and a lubricant. More particularly the present invention relates to an iron-based powder composition for powder metallurgy which causes less particle segregation of the additive and less generation of dust, and has excellent flowability and compactibility over a broad temperature range from room temperature to about 200xc2x0 C. The present invention relates also to a process for production of the iron-based powder composition and a process for production of a compact from the composition.
Iron-based powder compositions for powder metallurgy have been produced generally by mixing an iron powder as the base material, and an alloying powder such as copper powders, graphite powders, and iron phosphide powders, and, if necessary, a machinability-improving powder, and a lubricant such as zinc stearate, aluminum stearate, and lead stearate. The lubricant has been selected in consideration of its mixability with the iron powder and its removability in the sintering process.
In recent years, in powder metallurgy, sintered members are demanded to have higher strength. To meet the demand, a xe2x80x9cwarm compaction techniquexe2x80x9d has been developed in which powdery material filled in a metal die is compacted with heating at a certain temperature to obtain a compact having a higher density and a higher strength (See, for example, Japanese Patent Application Laid-Open Gazette (Kokai) No. Hei. 2-156002, Japanese Patent Publication (Kokoku) No. Hei. 7-103404, U.S. Pat. No. 5,256,185, and U.S. Pat. No. 5,368,630). The lubricant added to the iron powder in the warm compaction technique should have lubricity in the compaction process in addition to the above required properties. This lubricity is important to improve the compactibility by reducing frictional resistance between the iron powder particles and between the metal die and the formed compact by melting a part or the entire of the lubricant and dispersing it uniformly throughout the iron powder particle interspace. However, a conventional powder mixture is liable to cause particle segregation of an alloying powder or other additive disadvantageously. A powder mixture generally contains powder particles having various particle sizes, various particle shapes, and different particle densities, so that segregation tends to occur during transportation after the mixing, on charging into or discharging from a hopper, or during compacting.
For example, a mixture of iron-based powder and graphite powder is known to undergo particle segregation during truck transportation by vibration in a transporting vessel to separate graphite particles on the powder surface. A powder composition charged into a hopper undergoes segregation during movement within the hopper, causing variation of graphite powder content in the discharged powder composition from the initial stage to the end stage of the discharge. The final sintered articles produced from the segregated nonuniform powder composition are liable to vary in chemical composition, dimension, and strength, which can make the products inferior. The graphite powder or an additive, which is usually fine powdery, increases the specific surface area of the powder composition to lower the flowability of the composition. The lower flowability of the composition decreases the speed of filling the powder composition into a die cavity, lowering the compact production rate.
For preventing the segregation of the powder composition, addition of a binder is disclosed in Japanese Patent Application Laid-Open Gazette Nos. Sho.56-136901 and Sho.58-28321. However, a larger amount of addition of a binder to prevent the segregation in the powder composition poses another problem of fall of the flowability of the entire powder composition disadvantageously.
The inventors of the present invention disclosed use of a co-melted mixture of a metal soap or a wax and an oil as a binder in Japanese Patent Application Laid-Open Gazette Nos. Hei. 1-165701 and Hei. 2-47201. The disclosed binder reduces remarkably the segregation of the powder composition and the scattering of dust, and improves the flowability. However, this technique poses another problem of variation of the flowability of the powder composition with lapse of time owing to the above method of segregation prevention, namely the increase of the amount of the binder.
The inventors of the present invention disclosed use of a co-melted mixture of a high-melting oil and a metal soap as a binder in Japanese Patent Application Laid-Open Gazette No. Hei. 2-57602. This technique reduces deterioration with time of the properties of the co-melted mixture and deterioration with time of flowability of the powder composition. This technique, however, poses still another problem such that the apparent density of the powder composition changes because a high-melting saturated fatty acid in a solid state and a metal soap are mixed with the iron-based powder. To solve this problem, the inventors of the present invention disclosed, in Japanese Patent Application Laid-Open Gazette No. Hei. 3-162502, a method in which the surface of the iron-based powder particles is coated with a fatty acid, an alloying powder or a like additive is allowed to adhere thereto through a co-melted mixture of a fatty acid and a metal soap, and then a metal soap is added onto the outer surface thereof.
The above techniques disclosed in Japanese Patent Application Laid-Open Gazette Nos. Hei. 2-57602 and Hei. 3-162502 solve the problems of segregation in the powder composition and generation of dust to a considerable extent. With this technique, however, the flowability of the powder composition is insufficient: especially the flowability in xe2x80x9cwarm compactionxe2x80x9d in which the powder composition heated to about 150xc2x0 C. is filled in a hot die and is compacted. Further, the improvements of compactibility of the powder composition in warm compaction disclosed in Japanese Patent Application Laid-Open Gazette Nos. Hei. 2-156002, and Hei. 7-103404, U.S. Pat. Nos. 5,256,185, and 5,368,630 mentioned above are not sufficient in the flowability of the powder composition in warm compaction owing to liquid bridge formation by a low-melting lubricant component between particles. The insufficient flowability not only reduces the productivity of the compacts but also causes variation of the density of the compacts and variation of the properties of the final sintered products. Furthermore, the warm compaction technique disclosed in above Japanese Patent Application Laid-Open Gazette No. Hei. 2-156002, etc. enables production of iron-based compact having high density and high strength, but requires stronger ejection force for removal of the compact from the die and is liable to cause scratches on the compact surface or to shorten the life of the die.
The present invention intends to provide an iron-based powder composition for powder metallurgy excellent in flowability and compactibility in comparison with conventional ones at room temperature and in warm compaction, and intends also to provide a process for producing the powder composition, and a process for producing a compact having a higher density and a higher strength.
Flowability of metal powder is extremely impaired generally by addition of a lubricant or a like organic material. The inventors of the present invention made investigation on this problem, and found that frictional resistance and adhesive force between the metal powder and the organic material impairs the flowability. Therefore, the inventors made comprehensive study on reduction of the frictional force and the adhesive force, and found that the frictional resistance can be reduced by surface treatment (coating) of the metal powder particles with a certain organic material which is stable up to the warm compaction temperature (about 200xc2x0 C.), and that the adhesion by electrostatic force can be decreased by bringing the surface potential of the metal powder particles to the surface potential of the organic material (except the above surface treating material) to retard contact electrification between different kind of particles on mixing.
Further, the inventors of the present invention made investigation on solid lubricants for improvement of compactibility of a powder composition, and found that the force for removing a compact from a die after compaction (hereinafter referred to as ejection force) can be reduced to improve compact productivity by use of an organic or inorganic compound having a layer crystal structure in a temperature range from room temperature to warm compaction temperature, or by use of a thermoplastic resin or elastomer capable of undergoing plastic deformation at a temperature higher than 100xc2x0 C. in warm compaction. They also found that the coating of the metal powder surface with the above surface treating material for flowability improvement reduces secondarily the ejection force to improve the compactibility. The present invention has been accomplished on the basis of the above findings.
The present invention provides an iron-based powder composition for powder metallurgy having higher flowability and higher compactibility, comprising an iron-based powder, a lubricant, and an alloying powder, at least one of the iron-based powder, the lubricant, and the alloying powder being coated with at least one surface treatment agent selected from the group of surface treatment agents below:
Surface treatment agents
Surface treatment agents: organoalkoxysilanes, organosilazanes, titanate coupling agents, fluorine-containing silicon silane coupling agents.
The present invention provides also an iron-based powder composition for powder metallurgy having higher flowability and higher compactibility, comprising an iron-based powder, a lubricant fixed by melting to the iron-based powder, an alloying powder fixed to the iron-based powder by the lubricant, and a free lubricant powder, at least one of the iron-based powder, the lubricant, and the alloying powder being coated with at least one surface treatment agent selected from the group shown above.
The surface treatment agent selected from the above group may be replaced by a mineral oil or silicone fluid in the present invention. The mineral oil is preferably an alkylbenzene.
The iron-based powder as the base in the present invention includes pure iron powder such as atomized iron powder, and reduced iron powder; partially diffusion-alloyed steel powder; and completely alloyed steel powder. The partially diffusion-alloyed steel powder is preferably a steel powder alloyed partially with one or more of Cu, Ni, and Mo. The completely alloyed steel powder is preferably a steel powder alloyed with Mn, Cu, Ni, Cr, Mo, V, Co, and W.
The alloying powder includes graphite powders, copper powders, and cuprous oxide powders as well as MnS powders, Mo powders, Ni powders, B powders, BN powders, and boric acid powders. The alloying powder may be used singly or in combination of two or more thereof. Graphite powders, copper powders, and cuprous oxide powders are especially preferred since they increase the strength of the sintered article as the final product. The alloying powder is incorporated into the composition at a content ranging from 0.1 to 10 wt % relative to the iron-based powder (100 wt %), since the final sintered article has excellent strength at a content of 0.1 wt % or more of the graphite powder; a powder of a metal such as Cu, Mo, and Ni; or a boron powder, but impairs dimensional accuracy of the final sintered product at a content of higher than 10 wt %.
The aforementioned organoalkoxysilane as the surface treatment agent is a substance having a structure of R4xe2x88x92mSixe2x80x94(OCnH2n+1)m (where R is an organic group, n and m are respectively an integer, and m=1xe2x88x923). The organic group R may have a substituent or be not substituted. In the present invention, the organic group R preferably has no substituent. The substituent is preferably selected from the groups of acryl, epoxy, and amino.
The organosilazane includes those represented by any of the general formulas: RnSi(NH2)4xe2x88x92n, (R3Si)2NH, R3SiNH(R2SiNH)nSiR3, (R2SiNH)n, and R3SiNH(R2SiNH)nSiR3.
The lubricant in the present invention is a fatty acid amide and/or a metal soap. This lubricant prevents surely segregation of the iron-based powder composition and dust generation, and improves flowability and compactibility. The fatty acid amide is contained preferably at a content of from 0.01 to 1.0 wt %, and the metal soap is preferably contained at a content from 0.01 to 1.0 wt % based on the weight of the powder composition. The fatty acid amide includes ethylenebis(stearamide), and bis-fatty acid amides. The metal soap includes calcium stearate, and lithium stearate.
The lubricant also includes inorganic compounds having a layer crystal structure, organic compounds having a layer crystal structure, thermoplastic resins, and thermoplastic elastomers. The lubricant may be employed singly or in combination of two or more thereof. The inorganic compound having a layer crystal structure is preferably one or more of graphite, carbon fluoride, and MoS2. The organic compound having a layer crystal structure is selected from melamine-cyanuric acid adduct (MCA) and xcex2-alkyl-N-alkylaspartic acid. The thermoplastic resin is preferably one or more selected from polystyrene, nylon, and fluoroplastics in a powder state having a particle size of not more than 30 xcexcm. The thermoplastic elastomer is preferably in a powder state having a particle size of not more than 30 xcexcm. The thermoplastic elastomer is more preferably one or more materials selected from styrene block copolymer (SBC), thermoplastic elastomer olefin (TEO), thermoplastic elastomer polyamide (TPAE), and thermoplastic elastomer silicone. The fatty acid includes linoleic acid, oleic acid, lauric acid, and stearic acid.
The xe2x80x9cfree lubricant powderxe2x80x9d in the present invention exists in a simple mixed state without adhering to the iron-based powder or the alloying powder, and is contained in the iron-based powder composition in an amount preferably from 25% to 80% by weight based on the total weight of the lubricants added.
The above iron-based powder composition of the present invention is produced by the process described below. This process is also included in the present invention.
In a typical process for producing the iron-based powder composition for powder metallurgy having higher flowability and higher compactibility of the present invention by fixing an alloying powder by a molten lubricant onto an iron-based powder, the process comprises a first mixing step of mixing, with the iron-based powder and the alloying powder, two or more lubricants selected from the lubricants shown below to obtain a mixture; a melting step of stirring the mixture obtained in the first mixing step with heating up to a temperature higher than the melting point of one of the lubricants to melt the lubricant having a melting point lower than that temperature; a surface treating-fixing step of cooling with stirring the mixture after the melting step, adding a surface treatment agent in a temperature range from 100 to 140xc2x0 C., and fixing the alloying powder onto the surface of the iron-based powder by the molten lubricant; and a second mixing step of mixing at least one lubricant selected from the group of lubricants shown below with the mixture after the surface treating-fixing step.
Lubricants: fatty acid amides, metal soaps, thermoplastic resins, thermoplastic elastomers, inorganic materials having layer crystal structure, and organic materials having a layer crystal structure.
In the first mixing step in the present invention, preferably one or more lubricants are selected from the aforementioned group of the lubricants, and one of the lubricants is preferably a fatty acid amide. Alteratively in the first mixing step, one or more lubricants may be selected from the metal soaps and the above lubricants, and the aforementioned one of the lubricants may be a metal soap. Only one lubricant may be used in the present invention.
In another typical process for producing the iron-based powder composition having excellent flowability and compactibility of the present invention for powder metallurgy by fixing an alloying powder by a molten lubricant onto an iron-based powder, the process comprises a surface-treating step of coating the iron-based powder and the alloying powder with a surface treatment agent; a first mixing step of mixing, with the iron-based powder and the alloying powder after the surface-treating step, two or more lubricants selected from the lubricants shown above to obtain a mixture; a melting step of stirring the mixture after the first mixing step with heating up to a temperature higher than the melting point of one of the lubricants; a fixing step of cooling with stirring the mixture after the melting step, and fixing the alloying powder onto the surface of the iron-based powder by the molten lubricant; and a secondary mixing step of mixing at least one lubricant selected from the lubricants shown above with the mixture after the fixing step.
In this embodiment also, in the first mixing step, preferably the lubricants are selected from the aforementioned group of the lubricants, and the aforementioned one of the lubricants is preferably a fatty acid amide. Alteratively, in the first mixing step, the one or more lubricants are selected from the metal soaps and the above lubricants, and one of the lubricants is a metal soap. Otherwise, in the first mixing step, two or more lubricants are selected from fatty acids, fatty acid amides, and metal soaps, and the same lubricants are used in the second mixing step. Use of only one lubricant is acceptable also in this embodiment.
In the above production processes, one or more surface treatment agents are employed which are selected from organoalkoxysilanes, organosilazanes, titanate coupling agents, and fluorine-containing silicon silane coupling agents. The above surface treatment agent may be replaced by a mineral oil or silicone fluid. The weight ratio of the lubricant added in the second mixing step is preferably in the range of from 25% to 80% by weight based on the total weight of the lubricants added in the first and second mixing steps.
The process for producing a compact of the present invention is characterized in that any of the aforementioned iron-based mixture is compressed in a die and then the formed compact is ejected therefrom wherein the temperature of the iron-based powder composition in the die is controlled to be higher than the lowest of the melting points of the lubricants contained in the composition but is lower than the highest thereof.
The main constitutional requirements of the present invention are described above. The effects of the surface treatment agent and the lubricants on the flowability and the compactibility are described below in detail, which are the most important points of the present invention.
Generally, flowability of a metal powder is extremely impaired by addition of an organic material like a lubricant as described above. This is caused by high frictional resistance and strong adhesion force between the metal powder and the organic material. This problem may be solved by treating (coating) the surface of the metal powder with a specific organic material to reduce the frictional force and to retard electrostatic adhesion between the different kinds of particles by bringing the surface potential of the metal powder to that of the organic material (excluding the surface treatment agent of the present invention). In other words, the flowability of the powder composition can be improved by synergistic effects of lowered frictional resistance and the lowered contact electrification. Thereby, the flowability can be achieved stably to enable warm compaction in a temperature range from room temperature to about 200xc2x0 C.
The organic material used therefor in the present invention includes organoalkoxysilanes, organosilazanes, silicone fluids, titanate coupling agents, and fluorine-containing silicon silane coupling agents. Such an organic material, namely a surface treatment agent, has a lubricating function owing to its bulky molecular structure and is effective in a broad temperature range of from room temperature to about 200xc2x0 C. because of its stability at high temperatures in comparison with fatty acids, mineral oils, and the like. In particular, the organoalkoxysilane, organosilazane, titanate coupling agent or fluorine-containing silicon silane coupling agent undergoes condensation reaction by a functional group thereof with a hydroxy group existing on the surface of a metal powder to form chemical bonding of the organic material onto the surface of the metal powder particle. Thereby, the surface of the metal powder particles is modified, and the effect of modification is remarkable at high temperatures without separation or flowing-away of the organic material.
The organoalkoxysilane has an organic group or groups which may be unsubstituted or substituted by a group of acryl, epoxy, or amino, but unsubstituted one is preferred. The organoalkoxysilane may be a mixture of different ones. However, an epoxy-containing one and an amino-containing one should not be mixed since they react together to cause deterioration. The number of alkoxy group (CnH2n+1Oxe2x80x94) in the organoalkoxysilane is preferably less.
The organoalkoxysilane having an unsubstituted organic group includes methyltrimethoxysilane, phenyltrimethoxysilane, and diphenyldimethoxysilane. The one having an acryl-substituted organic group includes xcex3-methacryloxypropyl-trimethoxysilane. The one having an epoxy-substituted organic group includes xcex3-glycidoxypropyl-trimethoxysilane. The one having an amino group includes N-xcex2(aminoethyl)-xcex3-aminopropyl-trimethoxysilane. Of the above organoalkoxysilanes, the fluorine-containing silicon silane coupling agents are useful in which a part of the hydrogen atoms in the organic group are replaced by fluorine. The titanate coupling agent includes isopropyltriisostearoyl titanate.
The organosilazane is preferably an alkylsilazane. A polyorganosilazane having a higher molecular weight may be used.
In place of the above surface treatment agents, silicone fluid, or a mineral oil is useful in the present invention. The silicone fluid is bulky, and reduces frictional resistance between particles by adhesion onto the surface of the metal powder particles to improve flowability of the powder. This lubrication effect is given over a broad temperature range owing to its thermal stability. The silicone fluid useful as the surface treatment agent includes dimethyl silicone fluid, methylphenyl silicone fluid, methylhydrogen silicone fluid, methylpolycyclosiloxanes, alkyl-modified silicone fluid, amino-modified silicone fluid, silicone-polyether copolymers, higher aliphatic acid-modified silicone fluid, epoxy-modified silicone fluid, and fluorine-modified silicone fluid. The mineral oil is useful because it improves flowability of a powder and is thermally stable to give the lubricating effect over a broad temperature range. An alkylbenzene is preferred as the mineral oil, but is not limited thereto in the present invention.
The surface treatment agent is added to the iron-based powder composition in an amount ranging from 0.001 to 1.0 wt % based on treated powder (100 wt %). With the addition of less than 0.001 wt %, the flowability will become lower, whereas with the addition of more than 1.0 wt %, the flowability will become lower.
Next, the lubricant is explained below. The lubricant is incorporated into the powder composition for the following reasons. Firstly, the lubricant serves as a binder for fixing the alloying powder to the iron-based powder to prevent segregation of the alloying powder and generation of dust. Secondly, the lubricant promotes rearrangement and plastic deformation of the powder in the compaction process to increase the green density of the compact owing to lubrication action mainly in a solid state. Thirdly, the lubricant reduces frictional resistance between the die wall and the formed compact at the ejection of the compact from the die to decrease the ejection force.
For achieving such effects, the powder composition in the present invention is prepared by mixing the alloying powder and the lubricant into the iron-based powder, heating the composition at a temperature higher than the melting point of at least one of the lubricants, and cooling it. When only one kind of lubricant is used, the lubricant is melted. When two or more kinds of lubricants are used, one lubricant having a melting point of lower than the heating temperature is melted. The melted lubricant forms liquid bridges between the iron-based powder and the alloying powder or the unmelted lubricant near the iron-based powder particles to allow the alloying powder and/or the unmelted lubricant to adhere to the surface of the iron-based powder. By solidification of the melted lubricant, the alloying powder is fixed to the iron-based powder. For example, with two lubricants having respectively a melting point of 100xc2x0 C. and 146xc2x0 C., the composition may be heated to 160xc2x0 C. to melt the two lubricants, or may be heated to 130xc2x0 C. to melt one lubricant with the other lubricant kept unmelted.
If the heating temperature for melting the lubricant exceed 250xc2x0 C., oxidation of the iron-based powder proceed to lower its compactibility. Therefore, at least one lubricant has preferably a melting point lower than 250xc2x0 C. to conduct heating at a temperature lower than 250xc2x0 C.
In compaction of the iron-based powder composition, the lubricant as a binder promotes arrangement and plastic deformation of the powder. Therefore, the lubricant is desirably dispersed uniformly on the surface of the iron-based powder. On the other hand, ejection force on removal of the compact from the die is reduced by the lubricant existing in a solid state on the surface of the compact, the lubricant liberated from the iron-based powder surface, and the lubricant sticking onto the iron-based powder surface in an unmelted state during the preparation of the composition. The latter is more important.
For achieving both of the above effects simultaneously, the amount of the free lubricant existing in the interspace of the iron-based powder particles is adjusted to be in the range from 25% to 80% by weight based on the total amount of the lubricant. With the free lubricant of less than 25% by weight, the ejection force for removing the compact is not decreased, and scratches can be formed on the surface of the compact, whereas with the free lubricant of more than 80% by weight, the fixation of the alloying powder onto the iron-based powder is weak, causing segregation of the alloying powder to result in variation of the quality of the final sintered product. Incidentally, for increasing the free lubricant in the powder composition, the lubricant is supplementally added in the second mixing step.
The lubricant is preferably a fatty acid amides and/or a metal soaps, and additionally at least one material selected from inorganic compounds having a layer crystal structure, organic compounds having a layer crystal structure, thermoplastic resins, and thermoplastic elastomers is added preferably thereto. More preferably, a fatty acid is added into a fatty acid amides and/or a metal soaps.
The use of a material having a layer crystal structure reduces the ejection force required after the compaction, improving the compactibility. This is considered to be due to the fact that the material can readily be cleaved along the crystal plane by shearing force in the compaction to reduce the frictional resistance between the particles in the compact and facilitate slippage between the compact and the die. The inorganic material having a layer crystal structure includes graphite, MoS2, and carbon fluorides. A smaller particle size is effective for reduction of the ejection force.
The organic compound having a layer crystal structure includes melamine-cyanuric acid adduct (MCA), and xcex2-alkyl-N-alkylaspartic acid.
Further addition of a thermoplastic resin or a thermoplastic elastomer to the iron-based powder and the alloying powder reduces the ejection force in compaction, especially in warm compaction. The thermoplastic resin has lower yield stress at higher temperature, and is deformed readily by lower pressure. In warm compaction of a metal powder containing particulate thermoplastic resin by heating, the thermoplastic resin particles undergoes plastic deformation readily among the metal particles or between the metal particles and the die wall to reduce the frictional resistance between the metal faces.
The thermoplastic elastomer is a material having a mixed phase texture having a thermoplastic resin (rigid phase) and a rubber-structured polymer (flexible phase). With elevation of the temperature, the yield stress of the rigid phase of the thermoplastic resin decreases to cause deformation readily at a lower stress. Therefore, the particulate thermoplastic elastomer contained in the metal particles gives the same effects as the aforementioned thermoplastic resin in warm compaction. The suitable particulate thermoplastic resin includes polystyrene, nylon, polyethylene, and fluoroplastics. The thermoplastic elastomer has preferably a rigid phase of resins including styrenic resins, olefinic resins, amide resins, and silicone resins. Of these, styrene-acrylic copolymers, styrene-butadiene copolymers are preferred. The above thermoplastic resin or the thermoplastic elastomer has a particle size of not larger than 30 xcexcm, preferably in the range of from 5 to 20 xcexcm. With the particle size of larger than 30 xcexcm, the resin or elastomer does not dispersed sufficiently among the metal particles, not giving the desired lubrication effects.
Alternatively, the lubricant may be a fatty acid amide and/or a metal soap, and if desired further, a fatty acid may be incorporated. However, the fatty acid, which has generally a low melting point, forms liquid bridges by melting between the iron-based powder particles when exposed to a temperature higher than 150xc2x0 C., tending to lower the flowability of the powder composition. Therefore, it should be used at a temperature not higher than about 150xc2x0 C.
The last description on the lubricant is shown below. The lubricant is incorporated into the iron-based powder composition in a total amount ranging from 0.1 to 2.0 wt % based on the iron-based powder (100 wt %). At the lubricant content of less than 0.1 wt %, the compactibility of the powder composition will be lower, whereas at the lubricant content of more than 2.0 wt %, the green density of the compact produced from the powder composition will be lower to give lower strength of the compact. In the present invention, one or more lubricants selected from metal soaps and fatty acid amides are preferably incorporated as a part or the entire of the lubricant. The metal soap includes zinc stearate, lithium stearate, lithium hydroxystearate, calcium stearate, and calcium laurate. The metal soap is preferably incorporated at a content ranging from 0.01 to 1.0 wt % based on the iron-based powder composition (100 wt %). At the metal soap content of higher than 0.01 wt %, the flowability of the composition is improved, whereas at the content of higher than 1.0 wt %, the strength of the compact produced from the composition is lower. The aforementioned fatty acid amide is selected from fatty acid monoamides and fatty acid bisamides. The fatty acid amide is preferably incorporated into the iron-based powder composition at a content ranging from 0.01 to 1.0 wt % based on the iron-based powder composition (100 wt %). At the fatty acid amide content of higher than 0.01 wt %, the compactibility of the powder composition is improved, whereas at the content thereof higher than 1.0 wt %, the density of the compact is lower.
In the present invention, the surface treatment agent employed for the purpose of improving flowability also serves to decrease the ejection force of the compact in the compaction of the powder composition as a secondary effect. The mechanism thereof is described below.
In production of a compact from a powdery matter by warm compaction, since the density of the compact is high, the metal powder particles on the compact surface tend to adhere to a die wall by compaction pressure, thereby a large ejection force being required for removal of the compact from the die, and the compact surface being scratched. By preliminarily coating the metal powder surface with a surface treatment agent of the present invention, a coating film is formed between the die wall and the metal powder on the compact surface. Thereby the ejection force is reduced, and the scratching of the compact and other problems are solved.
The present invention also provides a process for producing a high-density compact from an iron-based powder composition by utilizing the above secondary effects.
The process for producing a compact uses the aforementioned iron-based powder composition of the present invention. In the process, the composition is filled in a die, and is compacted with heating to a prescribed temperature to obtain a high-density compact.
The heating temperature thereof is selected in consideration of melting points of two or more lubricants added in the first mixing step. Specifically, the temperature is set between the lowest melting point and the highest melting point of the lubricants. When heated to a temperature higher than the lowest melting point of the mixed lubricants, the melted lubricant penetrates uniformly into the interspace of the powder by capillarity, thereby arrangement and plastic deformation of the powder is effectively promoted in press compaction to increase the density of the compact. In this step, the melted lubricant serves as a binder for fixing an alloying powder to the surface of the iron-based powder. The lubricant of the higher melting point in an unmelted state is dispersed over the surface of the iron-based powder or exists free state in the powder composition during preparation of the powder composition.
The lubricant existing in a free state or in a unmelted solid state in the powder composition disperses in the gap between the die and the compact to reduce the ejection force for removal of the high-density compact formed by compaction from the die.
When the compaction is conducted at a temperature lower than the melting points of all of the lubricants, no lubricant is melted, thereby arrangement and plastic deformation of the powder not being caused; the lubricant in the powder particle interspace does not emerge on the surface of the compact, causing a lower density of the produced compact. On the other hand, when the compaction is conducted at a temperature higher than the melting points of all of the lubricants, no lubricant is in a solid state, thereby the ejection force for removal of the compact from the die being increased and the compact surface being scratched; and during the rise of the density of the compact, the melted lubricants in the interspace of the powder particles is driven out to the surface of the formed compact to form coarse voids to lower the mechanical properties of the compact. Accordingly, adjustment of the amount of the free lubricant or unmelted lubricant in a solid state and the amount of the melted lubricant is especially important in the present invention.
Incidentally, the inorganic compound having a layer crystal structure, the organic compound having a layer structure, and the thermoplastic elastomer as the lubricants have no melting point. For such kinds of lubricants, a thermal decomposition temperature or a sublimation-beginning temperature is taken in place of the melting point in the present invention.